The present invention relates to optical communication techniques.
In an unrepeated system, a signal is communicated between a transmitter and a receiver without using inline optical amplifiers whose electrical power source has to be supplied either in mid-link (as in terrestrial systems), or is carried by the same cable that carries the optical signal (as in ultra-long-haul submarine systems). It is often desirable to communicate the signal over the maximum distance possible while maintaining sufficient optical signal-to-noise ratio (OSNR) for detection.
Amplification strategies for unrepeatered transmission systems can use: (a) forward and backward Raman, (b) transmitter-pumped and receiver-pumped ROPAs, (c) transmitter-pumped and receiver-pumped ROPAs with dedicated fibers for pump delivery. Traditionally, forward or backward Raman amplifiers of arbitrary order have been used, as shown in FIG. 1A. In this case, optical pump(s) propagate in the same fiber core as the signal; the Raman effect causes power to be transferred from the optical pump(s) to the signal. Alternatively, it is possible to use remote optically pumped amplifiers (ROPA) (FIG. 1B). A ROPA is a lumped amplifier, typically an erbium-doped fiber (EDF). The optical pump(s) for the ROPA propagate in the same fiber core as the signal, and can be supplied from the transmitter and/or receiver. Since pump photons are only converted into signal photons at the amplifier, and not needlessly converted in a distributed manner along the transmission fiber when the signal's power is still high, a system using ROPAs can achieve longer reach than one using forward and backward Raman amplifiers. Nevertheless, some Raman power transfer between the ROPA pump and the signal is unavoidable as the ROPA pump is typically at around 1480 nm, which in presence of a signal in the C-band (˜1550 nm) will result in non-negligible Raman pump efficiency. Recently, it was proposed that separate pump delivery fibers can be used to deliver the optical pump(s) to the ROPA to avoid the undesired Raman power transfer between the ROPA pump(s) and the signal (FIG. 1C).